1. Field of Invention
The present invention relates to the production of optical fiber couplers. More specifically, it addresses the problem of producing a plurality of optical fiber couplers at the same time that all have the same operating characteristics.
2. Description of Related Art
Generally, an optical fiber coupler is produced by carrying out the following manufacturing steps: 1) removing the coating from plurality of optical fibers, 2) heating and welding the optical fibers integrally so that the optical fibers are tightly in contact with each other, and 3) further heating and extending them. The strands of optical fibers are minute glass bodies each having an outer diameter of 125 .mu.m. Therefore, high accuracy is required to produce an optical fiber coupler. Therefore, the productivity of optical fiber couplers has been so low that the optical fiber couplers have been expensive to produce.
To improve productivity, there has been proposed a method of producing a plurality of optical fiber couplers at the same time, as disclosed in Japanese Patent Unexamined Publication No. Sho. 63-205615, or Japanese Patent Unexamined Publication No. Hei. 1-120510, the subject matter of which are incorporated herein by reference as if fully recited.
When producing a plurality of optical fiber couplers en bloc, it is required that all of the couplers have the same operating characteristics. However, using conventional production methods, it has been difficult to manufacture couplers with good reproducibility while making their characteristics uniform.
Optical fiber couplers produced simultaneously by conventional methods were measured as to their degrees of coupling. Their characteristics turned out to be non-uniform. It was difficult to simultaneously produce optical fiber couplers which exhibited the same degree of coupling. To learn more, four optical fiber couplers were produced from tape-like four-core optical fiber units and their structures were inspected in detail. FIG. 7 shows a section of an example of a tape-like four-core optical fiber unit. Another example of a tape-like four core optical fiber unit is shown in FIG. 16, where the individual optical fibers are spaced from each other. As shown in FIG. 7, each unit is constituted by four optical fibers or strands. Each optical fiber strand has a core 21A and a clad 22A and covered with a protective coating 23A. The optical fiber strands are arranged in a plane and covered with a coating layer 24A so as to be integrated flatly.
When optical fiber couplers were manufactured from such units using conventional methods, it was found that the quantity of welding varied on a large scale, as shown in FIG. 8. In FIG. 8, the abscissa designates optical fiber strands of tape-like four-core optical fiber units in the order sequentially from the outer side, and the ordinate designates the quantity of welding between the respective optical fiber strands. The quantity of welding was expressed by the width W of a welded portion between two strands, as shown in FIG. 9. The variation of the quantity of welding was larger in the outer side strands than in the inner side strands, as seen in FIG. 8. From this result, it was concluded that the optical fiber strands were not heated uniformly since the quantity of welding depends largely on the temperature of the optical fiber strands which are being heated.
From further detailed inspection, it was determined that there was a problem in the flow of gas at the time of heating the optical fiber strands. The distance between optical fiber strands after removal of the protective coatings of a tape-like optical fiber unit is so narrow (about 125 .mu.m) that heating gas cannot flow between the optical fiber strands satisfactorily. It is impossible to heat all the optical fiber strands uniformly.
The heating process is further explained with reference to FIG. 5. Reference numerals 1aA, 1bA, 1cA and 1dA represent optical fiber strands of one tape-like optical fiber unit, 2aA, 2bA, 2cA and 2dA optical fiber strands of the other tape-like optical fiber unit, and 4A a gas burner. Each of the pairs of the corresponding optical fiber strands 1aA and 2aA, 1bA and 2bA, 1cA and 2cA, and 1dA and 2dA are made to contact tightly with each other, heated and welded by the gas burner 4A. When heating, gas does not flow into portions between the optical fiber strands satisfactorily. Mostly, it flows outside (around) all of the strands.
Therefore, the flow of gas strongly heats only the optical fiber strands which are disposed on the opposite ends, so that the heating temperature of the outer side optical fiber strands is higher than that of the inner side optical fiber strands, and the quantity of welding of the outer side optical fiber strands is larger than that of the inner optical fiber strands. This phenomenon is not limited to the case of using tape-like optical fiber units. In the case of tightly contacting and arranging single-core optical fibers, the distance between optical fiber strands is also about 125 .mu.m in the same manner to thereby bring about such a phenomenon.